1. Field of the Invention
The invention relates to a cryogenic freezer for individually quick freezing articles such as food products. More particularly, the invention relates to a freezer designed to produce high quality quick frozen food products by placing the food products on a conveyor and delivering liquid and gaseous cryogen directly to the product on the conveyor.
2. Description of the Related Art
Individually quick frozen (IQF) products refer to food products which are frozen individually prior to packaging, as opposed to products which are frozen in clumps or blocks. Examples of such IQF products include hamburger patties, fruits, vegetables, diced ham, chicken pieces and patties, shrimp and scallops. Products frozen by IQF freezing techniques offer distinct advantages over block freezing methods. In particular, products which are frozen in clumps or blocks must be completely defrosted before any of the product can be used due to the fact that it is all frozen together. In contrast, IQF products can be used in the amount desired and the unused portion can be returned to the freezer. Due to this advantage, IQF products effect a premium on the market over block frozen products.
Different types of apparatus are known to be used for IQF freezing including straight tunnel freezers, mechanical chilled fluidized bed freezers, liquid nitrogen immersion freezers, carbon dioxide flighted freezers, and carbon dioxide tumbling freezers.
A traditional means of cryogenic freezing is through the use of what is referred to as a straight tunnel freezer. This process uses a conveyor belt that runs through an enclosed freezer. The product travels on the belt through the freezer. As the product travels, it is sprayed directly with cryogen and is cryogenically frozen. Typical cryogens used are liquid CO.sub.2 (-110.degree. F., -79.degree. C.) and liquid nitrogen (-320.degree. F., -196.degree. C.). The large difference in temperature between the product and the gaseous atmosphere in the freezer causes a quick heat exchange and results in fast freezing of the product. Because of this quick freeze, or cryogenic freeze, it is known that there is less cell damage in the product and thus less product damage. In the straight tunnel freezer, there are usually several top mounted fans blowing down on the product causing the thin layer of warm air surrounding the product to be removed and replaced with the colder cryogen gases. This results in a quick transfer of heat from the product. This process works well with products that are large, mostly dry, and can be placed on a single belt separated from each other so that they cannot freeze together. There are also so-called triple tier models, wherein food travels on three separate horizontal conveyer belts, the food on the top belt falling onto the middle belt, and then from the middle belt onto the bottom belt. Because the top-mounted fans cannot adequately move the gas past the food on the middle conveyer and especially the bottom conveyer, side-mounted fans are sometimes provided. However, these side-mounted fans are low power, simply to move the gas around, and are not intended to move the food on the conveyer belts or lift the food off of the belts.
One drawback for obtaining IQF quality with a straight tunnel freezer is that a small sized product will be blown off the belt by the fans and then will be lost in the freezer. Furthermore, there is also a problem with the product freezing to the belt, especially if the product is wet.
The mechanical chilled fluidized bed freezers employ a freon or ammonia cooling system for cooling the air within the freezer through which the food products pass on a conveyor. The food product is levitated by using fans or blowers which force mechanically refrigerated air upward through the conveyor belt. This creates a "fluidized" bed of cold air around the product which promotes heat transfer and thus individually quick freezes the product. Advantages of mechanical freezing are the low operating cost, high production rates, and efficiency due to recirculation of cold gases.
The main drawbacks of the mechanical fluidized bed freezer include the fact that product needs to be lightweight, of uniform shape, and small size for the true "fluidization" to occur. In addition, the complexity of the system usually requires special attention for installation and maintenance. Furthermore, frequent downtime is required to defrost the ice build-up on condensation coils inside the mechanical freezer. Other drawbacks include high capital investment, inflexibility of increased or decreased production requirements, and clumping if the product is not properly fed into the freezer.
A typical liquid nitrogen immersion is a relatively compact device which provides freezing quickly and in a short distance by direct immersion of food product in liquid nitrogen. With this freezer, the product travels on a belt in a downward inclined direction into a bath of liquid nitrogen where the partial fluidization and crusting of the product immediately take place. The product then travels in an upwardly inclined direction out of the liquid nitrogen. This is the fastest form of individual quick freezing available. If the product particles are not touching each other when freezing, the product will be individually quick frozen. In addition, products can be of varying sizes and shapes and little floor space is required for this freezer because of the low operating temperature. Efficiency of the system can be improved by adding a post-cool tunnel to extract heat from the unused nitrogen vapors. A post-cool tunnel is a straight tunnel that uses the cold vapors being exhausted by the immersion freezer and recirculates them around the product, further cooling the product.
There are several drawbacks with the liquid nitrogen immersion freezer. One drawback is that some products can crack from the drastic initial surface shrinkage of the product when immersed in liquid nitrogen. Also, due to the direct immersion into liquid nitrogen, it is very difficult to control the exit temperature of the product due to unpredictable volatility of liquid nitrogen. An additional drawback is that if the product particles are touching when immersed, they will be frozen together in clumps and not individually frozen. This freezing method does not use the chilling potential of the gas vapors and thus significantly decreases the efficiency. However, if a post-cool tunnel is used to increase the efficiency of the system, a substantial amount of additional floor space and capital cost are required.
A carbon dioxide flighted freezer typically provides a series of conveyor belts covered with a thin bed of CO.sub.2 snow. At the entrance end of the flighted freezer, there is a crust freezing zone where the product is sprayed with liquid CO.sub.2. The sprayed CO.sub.2 also creates a bed of dry ice snow in which the product travels along through the freezer. This bed of dry ice sublimes to help further freeze the product. The freezer tunnel has a series of belts which run at upward inclines. These inclined belt segments successively drop the food onto the next lower belt segment, dropping and tumbling the product and CO.sub.2 snow together through the freezer. The purpose of these successive drops is to break apart product that is frozen together. Unfortunately, this is not always successful and the impact created from the drop does not always successfully separate the product that has clumped together. The fans above the belt attempt to remove heat from the product and sublime the CO.sub.2 before it exits the freezer.
Additional drawbacks of the CO.sub.2 flighted freezer include the fact that CO.sub.2 is the only expendable refrigerant that can be used with this process. Excess solid CO.sub.2 snow on the belt that does not sublime before it exits the freezer may be packaged with the product. This packaged CO.sub.2 can expand in the package causing the package to burst. Product breakage is also a common occurrence in this freezer due to the tumbling action of the product. Inefficiency and inconsistent product quality are additional drawbacks of the carbon dioxide flighted freezer. In addition, the amount of floor space and equipment required to produce large production in a flighted freezer can be a disadvantage when space constraints and equipment costs at a production facility are a consideration.
A known carbon dioxide tumbler includes a long rotating cylindrical drum set on an angle and fed by a conveyor belt which loads product directly into the rotating drum. Baffles inside the rotating drum lift and spill the product and direct the product towards the exit end of the freezer. Liquid CO.sub.2 is also injected into the tumbler near the entrance and provides a tumbling bed of snow for the product to travel in. The disadvantages of this method are similar to the disadvantages of the CO.sub.2 flighted freezer. Additionally, due to very little shearing action within the process, product may stick and accumulate on the inner surfaces of the drum. Furthermore, the cool CO.sub.2 gas vapors are not used efficiently due to the lack of ventilated gas movement.